Azure

An application in an environment reaches Azure Blob Storage, Queue Storage and Table Storage without knowing it. It builds https://youraccount.blob.core.windows.net the way it does in production, that name resolves to the environment’s sidecar, the sidecar terminates TLS with a certificate authority the environment already trusts, and Azurite answers.

There is no endpoint override, no BlobEndpoint= in a connection string that only exists in tests, and no client construction that differs from the one that ships. That is the whole point of this page. The emulator is a commodity; reaching it without changing the application is not.

The surface

This table is the surface. A host outside it is not routed to an emulator: it falls through to the environment’s egress policy, whose default is block, so it is refused rather than answered. A silent wrong answer from an emulator is worse than a refusal, because it will be believed.

Service	Host	Emulator	Answered by
Azure Blob Storage	*.blob.core.windows.net	azure-blob	Azurite
Azure Queue Storage	*.queue.core.windows.net	azure-queue	Azurite
Azure Table Storage	*.table.core.windows.net	azure-table	Azurite

Azurite is published by Microsoft and is MIT licensed. It is the only one of the three Microsoft Azure emulators that is open source, and that difference decides more of this page than weight does.

Why three emulators for one image

Azurite is a single container that listens on three ports: blob on 10000, queue on 10001 and table on 10002. An emulator declaration carries one port, so blob, queue and table are registered separately against the same image.

The shape turned out better than the constraint that produced it. An environment starts the emulators its egress rules name, so a manifest that touches blob only starts one container and pays for one, and a refusal is written per service: a request to Azure Files is refused with Files named, rather than with “Azure” named.

The account name travels in the host

Azure puts the storage account in the first label of the hostname, so youraccount.blob.core.windows.net carries the account the way a virtual hosted S3 URL carries the bucket. The sidecar rewrites the destination and preserves the Host header, and Azurite reads the account out of that header when the host is a name rather than an address.

So nothing needs to tell Azurite which account it is serving through a flag. AZURITE_ACCOUNTS is deliberately not set, because the account an environment needs is whichever one its substituted credential names, and that is a property of the manifest rather than of this build.

The credential is substituted, not passed through. A request signed with a key the environment recognises as live is refused before it leaves, and the emulator has no route out in any case: it attaches to the environment’s inner network, which is created internal, so having nowhere to send a credential is a property of the network rather than a promise.

What it costs per environment

Measured on 2026-09-08 on an Apple Silicon machine, 8 core, with Docker Desktop holding 7.654 GiB, at load averages between 24 and 28, because the machine was running other work at the same time. The load is published with the numbers rather than left out: the memory figures are stable under it and the start times are not, and saying which is which is worth more than a best case.

Image sizes are compressed download bytes for the linux/arm64 member, read from the registry.

Container	Image	Resident	Started
azure-blob	108.9 MiB	69.9 MiB	bound all three ports
azure-queue	the same image, nothing more on disk	66.7 MiB	bound all three ports
azure-table	the same image, nothing more on disk	67.1 MiB	bound all three ports
all three	108.9 MiB once	203.8 MiB

One Azurite measured alone was 83.5 MiB, so the marginal cost of the second and third is about 67 MiB each. An environment that names only blob pays 69.9 MiB and one image.

That is the whole cost of Azure Blob, Queue and Table in an environment: one image and about 200 MiB of memory, or a third of that for one service.

What Service Bus would have cost, which is why it is not here

Container	Image	Result
servicebus-emulator	81.7 MiB	halted: SQL Health Check failed
mssql/server companion	595.9 MiB, AMD64 only	killed after 707 seconds, never ready

677.6 MiB of image before either process answers anything, against 108.9 MiB for all of Azurite. The SQL Server companion was given a 3 GB allocation of its own and was killed by the memory limit after 707 seconds, having reached TLS initialisation and no further; it never printed SQL Server is now ready for client connections, and the Service Bus emulator beside it then failed its SQL health check and halted. Under emulated AMD64 on ARM64 this is what the pair does on a developer laptop.

The load average was 25 to 26 throughout, on a shared machine, so this does not prove SQL Server cannot start here. It does mean the pair is in a different class of weight from Azurite by roughly an order of magnitude in image bytes and more than that in memory, and that a developer with an Apple Silicon machine who names Service Bus in a manifest would be waiting on an emulated SQL Server rather than testing their application. Opt in is the right answer even before the EULA below.

Cosmos DB

cosmosdb/linux/azure-cosmos-emulator:vnext-preview has a real ARM64 build and needs no companion. It is 645.6 MiB of image, six times Azurite, and held 89.9 MiB resident. Its readiness was NOT measured: the predicate used to watch for it matched the word ready inside its own retry line readiness check still waiting for Postgres startup, so the 97 seconds it reported is not a start time and is not published as one. Its own health line still read PostgreSQL=FAIL, Gateway=FAIL, Explorer=FAIL at that point.

Not answered, and why

Naming a service and not building it is worse than leaving it out, so these are named here rather than discovered in a failure.

Azure Service Bus

Microsoft publishes an emulator for it and this build does not start one, for two measured reasons and one that is not about cost at all.

Its wire protocol may not reach an emulator at all. Every Azure Service Bus SDK defaults to AMQP 1.0 on port 5672, which is not HTTP. A rule in emulate mode makes the sidecar terminate the connection and read an HTTP request out of it, so an AMQP connection would be dropped rather than forwarded. Service Bus also speaks HTTP on 5300, and that path would work.

This is the reason that would matter most, and it is NOT CONFIRMED BY EXPERIMENT. It was reasoned from policy.inspectMode and the sidecar’s http.ReadRequest by the lane that owns the routing, and neither that lane nor this one has driven an AMQP client at an emulate rule. It is recorded here because a reader deciding whether to wait for Service Bus deserves to know the strongest argument against it exists, and recorded as unconfirmed because it has not been run.

It cannot be declared yet. mcr.microsoft.com/azure-messaging/servicebus-emulator requires a SQL Server container beside it, which it dials on startup, and an emulator declaration carries one image. Nothing here can express a companion.

It refuses to start until somebody accepts a EULA. Started bare, with no environment set, it exits with code 133 in 47 seconds and says so: the Service Bus emulator EULA has to be accepted through ACCEPT_EULA=Y. That is an acceptance a user makes, not one a build makes on their behalf, which is a better reason for it to be opt in than any number on this page.

Its companion has no ARM64 build. mcr.microsoft.com/mssql/server:2022-latest is a single AMD64 manifest with no ARM64 member, so on an Apple Silicon machine Service Bus drags an emulated AMD64 SQL Server into every environment that names it. Measured above: 677.6 MiB of image, and the SQL Server never became ready in 707 seconds with 3 GB of its own.

Azure Cosmos DB

The Linux emulator is a single image with a real ARM64 build and no EULA gate, and its cost is above. It is not registered here because nothing in the conformance suite proves it, and a service in the surface that nothing proves is a claim rather than a capability. At 645.6 MiB it is also six times Azurite, so if it lands it lands opt in.

Everything else Azure runs

Azure Files (*.file.core.windows.net), Data Lake Storage Gen2 (*.dfs.core.windows.net), Key Vault (*.vault.azure.net), Event Hubs and the management plane are outside the surface. So are the sovereign cloud suffixes core.chinacloudapi.cn and core.usgovcloudapi.net, which are different hosts and are refused rather than answered.

Two of these are worth calling out because a reader is likely to expect them to be covered by something that is. A Service Bus queue is not a Queue Storage queue, and the Cosmos DB Table API is not Table Storage: it speaks the same protocol on table.cosmos.azure.com but its partitioning and throughput behaviour is what a Cosmos user is testing, and Azurite is not that.

Declared storage resources are not created in the twin, and a run says so

af up creates the cloud resources production’s infrastructure as code declares inside the emulators, before any service starts. It does not do that for Azure, and this is where a reader finds that out rather than from a twin that is quietly missing a container.

Azurite validates the Shared Key signature on every request. A request to create a blob container, addressed the way an environment addresses one, is refused:

PUT /afprobe?restype=container
Host: devstoreaccount1.blob.core.windows.net
x-ms-version: 2021-08-06

HTTP/1.1 403 Server failed to authenticate the request.
x-ms-error-code: AuthorizationFailure

Signing needs the storage account’s key, and the account credential an application receives is a substituted credential the manifest decides, so nothing in the engine holds one to sign with. LocalStack and both Google emulators answer an unsigned request, which is why those are seeded and this is not.

So azurerm_storage_container, azurerm_storage_queue and azurerm_storage_table are reported as unmeasured, each carrying that reason, and the emulator itself is still started and still answers the application. The container the application expects is the thing that is missing, and a run names it.

One trap is worth recording for whoever closes this. With production style addressing the account is in the hostname, and Azurite then refuses a path that also names the account, with a bare 400 and an empty body. The path is /<container> and not /devstoreaccount1/<container>.